An image sensor is a semiconductor device that converts optical images to electrical signals. Image sensors can be classified into a charge coupled device (CCD) image sensor or a CMOS image sensor. The CMOS image sensor sequentially detects electrical signals of each unit pixel using the switching mode to realize images by forming photodiodes and MOS transistors in the unit pixels. Such a CMOS image sensor has a structure in which photodiode areas for receiving and converting a light signal into an electrical signal and transistors for processing this electrical signal are horizontally arranged on and/or over a semiconductor substrate. By a horizontal CMOS image sensor, photodiodes and transistors are horizontally formed adjacent to each other on and/or over a substrate, thus requiring additional areas for forming photodiodes.